The present invention relates to a method of controlling primary measures for reducing the formation of thermal nitrogen oxides in gas turbines, the primary measures being controlled according to a characteristic which characterizes an air-quantity-specific heat release rate in a combustion chamber of a gas turbine. In particular, it relates to a method of controlling the fuel distribution to burners or burner groups of a gas turbine and of controlling the quantity of steam or water introduced into the combustion chamber of a gas turbine for reducing the nitrogen oxides.
Premix burners with which a very low pollutant burden, in particular a very low nitrogen-oxide burden, of the exhaust gas can be realized by lean premixed combustion without further measures, have been disclosed previously, for example by EP 0 321 809.
Burners of the type of construction as specified in the abovementioned publication have proved successful in gas turbines in practice in large quantities.
The burners referred to react in a very sensitive manner to changes in the stoichiometry of an individual burner: if a burner is operated with a rich mixture, the emissions increase exponentially. In the case of an operation on a lean fuel mixture, the flame rapidly becomes unstable. In fact, these burners in modern gas turbines are already operated very close to their stability or extinction limit. However, the overall stoichiometry of a gas turbine varies within wide limits over the entire operating range.
Gas turbines are therefore equipped with a multiplicity of burners whose fuel feed can be controlled individually or in groups. Thus, in the case of variable stoichiometry of the entire machine, the stoichiometry of the burners is kept within the operating limits. The nitrogen-oxide emissions are used as a measure of the burner stoichiometry, this measure manifesting itself externally, and although these nitrogen-oxide emissions may well be dependent on a number of other factors, they nonetheless provide a good indication of the flame temperatures, and ultimately also constitute a variable which can be kept below a limit value precisely by means of the burner technology described.
There are essentially two ways of keeping the burners at a reliable stoichiometric interval when the machine stoichiometry varies greatly overall. In one variant, the feed line of each burner is provided with a shut-off member. In the part-load range of the gas turbine, only some of these shut-off members are opened, and only some of the burners are operated. When the load is increased, more fuel is fed, which means a hotter flame when the air quantity stays the same. A point is reached at which the operation of a further burner is possible orxe2x80x94on account of exceeding admissible emissionsxe2x80x94is necessary. The entire fuel quantity is distributed to a larger number of burners by opening the shut-off member in the fuel feed line of a further burner, and thus each individual burner is operated on a leaner mixture again, that is, with a cooler flame and lower nitrogen-oxide emissions.
A further method consists in combining a plurality of burners in groups in each case. Thus a number of burners may be combined in two groups, of which the first comprises, for example, five or three times as many burners as the second. All the burners of each of these groups are connected to a common fuel line, in which a control member is arranged in each case. Within the lowermost load range, that is to say at the smallest fuel quantities, the first burner group is in operation, and the control member of the second burner group is completely closed. If need be, the flame stability is additionally assisted by a number of diffusion burners. With increasing fuel quantity, the diffusion burners may be shut down. With increasing fuel quantity, the burners of the first burner group are operated with an increasingly richer mixture. The control member in the fuel line of the second burner group is then successively opened, and some of the fuel is directed to the burners of the second group.
The introduction of inert media, preferably water or steam, into the flame or the flames in the interior of a gas-turbine combustion chamber offers a further possibility of reducing the formation of nitrogen oxides. In this case, the water or steam quantity is controlled as a function of a control variable in such a way that it has a certain ratio to the fuel quantity, this ratio being predetermined as a function of the control variable.
Irrespective of how the control of the fuel distribution or of the water and steam injection is executed, an appropriate variable has to be used for controlling the fuel distribution or the water and steam injection. Control, for example with the aid of measured emission data, proves to be problematic in transient operating states. The use of emission measurements with the requisite high accuracy and reliability over a long period also requires regular competent maintenance. A machine characteristic is thus used in practice as a parameter for controlling the fuel distribution. In test series, a suitable profile of the fuel supply is determined as a function of this variable and programmed in the control of the machine. The burner control therefore reacts directly to changes in the control parameter.
Such a parameter must reproduce the fuel/air ratio or the air-mass-specific heat release rate of the machine as accurately as possible, if need be weighted with some other variables which contain the flame stability and the formation of nitrogen oxides.
A control parameter with which it is appropriate to begin is the relative power output of a gas turbine, which on the one hand takes into account the power output of the gas turbine and on the other hand also takes into account changes in the air quantity on the basis of variable ambient conditions. However, it has been found that the relative load to a much greater extent represents a process variable rather than a combustion variable. On account of a multiplicity of factors, a change in the ambient temperature in particular is overcompensated for. Furthermore, in machines which have adjustable guide rows at the compressor inlet, the air quantity depends to a great extent on the position in particular of a preliminary guide row. Precisely in combined-cycle plants, however, this preliminary guide row is adjusted as a function of certain process temperatures. The position of the preliminary guide row and thus the air quantity of the combustion may therefore vary considerably at one and the same relative load at different ambient temperatures.
With the demands for efficiency and environmental compatability, which of necessity are to be made on the energy supply of the future, the reliable operating range within which the burners ensure compliance with the emission regulations at an adequate distance from the operating limit will become increasingly narrow, and such tolerances may become less and less acceptable. The statement that the operating range which is predetermined by statutory, technical and economic limits will be rapidly restricted also applies to other primary measures which are taken in order to reduce the formation of thermal nitrogen oxides.
An object of the invention is to provide a method of controlling primary measures for reducing the formation of thermal nitrogen oxides in gas turbines, the primary measures being controlled according to a characteristic which characterizes an air-quantity-specific heat release rate in a combustion chamber of the gas turbine. When this variable is being formed, recourse is to be had to data which are available in a reliable manner and where possible without a time delay.
According to the invention, this is achieved in that, in order to approximately determine the heat release rate, the power output available at an output-side end of a useful-power shaft of the gas turbine is used, and the air quantity is determined as a function of ambient conditions and machine operating data. In the case of the ambient conditions, in particular the compressor inlet temperature and the ambient air pressure, which determine the density of the air drawn in by the compressor, are to be taken into account. In addition, the air humidity may also be important. As far as the machine operating data are concerned, consideration is to be given first of all to the position of adjustable compressor guide rows and especially to an adjustable preliminary guide row, which factors influence the volumetric intake flow of a compressor to a very high degree.
With regard to the primary measures for reducing the formation of thermal nitrogen oxides, consideration in this context is to be primarily given to the distribution of the fuel quantity to different premix burners or groups of premix burners, or to the control of the quantity of inert media, for example steam or water, which is injected into a combustion chamber in order to reduce the flame temperature. In addition, a number of further methods for reducing the formation of thermal nitrogen oxides are of course also conceivable, which methods can be controlled by means of the method according to the invention.
Furthermore, it is advantageous to take into account the change in the air mass flow as a function of the position of the compressor guide rows, in particular of the preliminary guide row. It is also appropriate to correct the characteristic thus determined with a term which includes the pressure ratio of the gas turbine. Such a term must on the one hand take into account the fact that the ideal process efficiency increases with increasing pressure ratio, thus the ratio of the generator power output and the heat release rate in the combustion chamber changes. A further point which is advantageously taken into account by such a term is the change in the component efficiencies, in particular in the compressor efficiency, at a different pressure ratio. The compressor final temperature, which likewise depends on the pressure ratio to a great extent, influences the formation of nitrogen oxides as well as the flame stability. Likewise, the distribution of cooling air and combustion air also changes with the pressure ratio within certain limits. On account of the complex relationships, the pressure-dependent correction term is to be determined partly empirically or is to be adapted to the specific conditions of a machine.
In order to be able to utilize the characteristic in a practical manner, it is not necessary to determine this in a quantitatively exact manner. On the contrary, it is sufficient, starting from a certain reference point, to correctly reproduce the relative changes as a function of the machine data and ambient data cited.
It is of course also easily possible to use the characteristic thus determined to control the water or steam injection into the combustion chamber of a gas turbine as described briefly above.